This invention relates to a method for inhibiting the corrosion of ferrous metals. More particularly, this invention concerns a method for inhibiting corrosion of a ferrous metal subjected to aqueous acid environments. In narrower aspects, the present invention relates to an acidizing process for the removal of calcareous scale formations from a ferrous metal surface whereby corrosive attack of the metal substrate is inhibited.
In many industrial practices, the need to contact ferrous metal surfaces with strong acidic agents constantly arises. Particularly under numerous industrial circumstances equipment fabricated of ferrous metals such as boilers often accumulate scale formations which must be periodically removed in order to maintain the efficiency of the equipment involved. For example, it is common within the petroleum industry to utilize heater tubes in contact with crude oil containing emulsified aqueous solutions of inorganic salts, particularly those of calcium, in order to effect separation of oil and water. The briny solution, upon contacting the heated metallic surfaces, deposits a substantial portion of mineral content upon said surfaces in a form of insoluble calcareous formations. Consequently, in order to maintain reasonable efficiency of the heaters and to obviate burning out of the heater tubes it is necessary to remove the scale formations periodically that have formed. The latter is but one example of many of such operations in the petroleum field which requires frequent acidizing treatment of various pieces of equipment made of iron or a ferrous alloy. Also in many chemical processes it is necessary to store or transport acids, such as the common mineral acids, while in contact with ferrous metals. Accordingly, it can be seen that the practical situations necessitating the minimizing or obviating of the corrosive effect acidic materials, especially those of mineral origin, upon ferrous metal surfaces are legion.
Flow-induced localized corrosion (FILC) is a result of high shear conditions present in flow lines. The amount of corrosion that occurs is dependent on a variety of factors including the corrosiveness of the fluid flowing through the lines, the metallurgy of the line and the ability of added corrosion inhibitors to maintain adhesion to the interior of the line.
The ability of added corrosion inhibitors to maintain adhesion to the interior of the line depends on both the chemical adhesive properties of the inhibitor and the shear stress conditions which exist inside the line. A number of products have shown promise as shear-resistant corrosion inhibitors. Included among the corrosion inhibitors are amides and quaternized amines and amide amine salts.
Many nitrogen-containing corrosion inhibitors are known. Among them are: morpholine, disclosed in U.S. Pat. No. 3,649,167; imidazolines in U.S. Pat. No. 3,197,403; amidic acids in U.S. Pat. No. 2,742,498; bis-amides in U.S. Pat. No. 4,344,861; amines in U.S. Pat. Nos. 5,211,840 and 5,714,664; amides in U.S. Pat. No. 5,556,575; and carbohydrazides in U.S. Pat. No. 5,078,966. The reaction of acrylic acid with substituted imidazolines to form various amine derivative corrosion inhibitors has been disclosed in U.S. Pat. Nos. 5,300,235; 5,322,630; 5,322,640 and 5,427,999.
Schiff bases have been disclosed as corrosion inhibitors in U.S. Pat. No. 5,147,567, in Materials Chemistry and Physics. 39 (1995) 209-213 and in Corrosion Science, 26 (1986) 827-837. Moreover, imines have also been disclosed as corrosion inhibitors, and as additives to corrosion inhibitor formulations. For example, imines have been disclosed as useful in conjunction with aromatic triazoles to enhance corrosion inhibition in U.S. Pat. No. 4,642,221; and imine dispersants have been utilized to enhance the activity of trithiones as CO.sub.2 corrosion inhibitors in EP 0 275 651. Imines have been disclosed as corrosion inhibitors in U.S. Pat. No. 4,490,293; JP 74021024B and SU 318315A. However, none of these references recognize the corrosion inhibiting characteristics of zwitterionc imines, which I have discovered.
It is one of the objects of this invention to provide a process whereby ferrous metal surfaces can be subjected to corrosive environments without the occurrence of an untoward degree of corrosion.